C18 | Assessment of bioaccumulation and depuration of PET microfibres in specimens of Mytilus galloprovincialis: preliminary study

Purpose. The aim of the study, conducted as part of the PRIN 2020-2022 project “PLASTACTS: Assessment of nano/microplastics impacts”, was to evaluate changes in the accumulation and purification capacity of Mytilus galloprovincialis after exposure to different concentrations of polyethylene terephthalate (PET) microfibres (MF) under controlled conditions. Methods. Specimens of M. galloprovincialis, collected from a purification centre in Messina (Italy), were acclimatised for seven days in the laboratory, in aquariums with filtered seawater (15 °C), equipped with filters, skimmers, oxygenators and fed daily with a commercial microalgae suspension. After this phase, the mussels were fixed on polylactic acid (PLA) supports and transferred to contamination tanks containing filtered seawater at 15 °C equipped exclusively with an oxygenator. They were exposed for 24 hours to PET MF (≤200 µm; Ø 18 µm) at four concentrations: 100, 10, 1 and 0.1 MF/ml. For each concentration, three replicates of 10 specimens each (total 30 mussels) were prepared; in parallel, two mussels per replicate (six in total) were used as controls to exclude environmental contamination during the analyses. The MF were produced at the IPCB-CNR laboratories by cryogenic ultracentrifugation of textile residues washed without detergent. After exposure, the specimens were rinsed and transferred to purification tanks with filtered (15 °C) and oxygenated seawater. At 24 and 48 hours, the bottom of the tanks was siphoned to collect the excreted MF, incorporated in faeces or pseudofaeces. The water samples were filtered through cellulose membranes (0.45 μm) and analysed under a stereomicroscope to count the MF. After 48 hours, the mussels were digested with 10% KOH (1:40 w/v); the digested material was filtered and analysed in the same way as the water samples. Results. After 24 hours, the number of excreted MFs showed no correlation with the tested exposure concentration (11 MFs at 0.1 MF/ml; 42 at 1 MF/ml; 24 at 10 MF/ml; 47 at 100 MF/ml). After 48 hours, excreted MFs increased, but still without a clear relationship to concentration (29 MFs at 0.1 MF/ml; 38 at 1 MF/ml; 97 at 10 MF/ml; 94 at 100 MF/ml). The MF retained in mussels after 48 hours also showed no correlation (16 MF at 0.1 MF/ml; 20 at 1 MF/ml; 7 at 10 MF/ml; 13 at 100 MF/ml). Conclusions. These results indicate that the accumulation and depuration of MF in mussels may not be directly dependent on exposure concentration. It is more likely that other factors, such as particle size, shape and spatial arrangement, have a greater influence on these processes. Considering that most studies use virgin plastic fragments, often at unrealistic concentrations, and not fibres, the most common form of microplastics in the environment, the use of real MF at ecologically plausible concentrations is a more realistic approach and provides more representative data for the study of bioaccumulation and depuration mechanisms in mussels.